Gas discharge lamps are well known in the art. Typically, such lamps are energized by a ballast. Unlike incandescent lights, gas discharge lamps and their accompanying ballasts as found in the prior art do not switch on instantly. When turn on time becomes too long, users of the product may become confused when trying to switch the light on, and may conclude that the light or the ballast is no longer functioning properly.
An electronic ballast has a boost coupled to an inverter. The output of the inverter energizes the lamps. Before the lamps are fully energized, the boost and the inverter must begin to operate. This creates a delay which, if not controlled, is perceptible to the user.
Some electronic ballasts have a boost circuit. The boost circuit provides power factor correction, as is well known in the prior art. The boost is composed of a bridge rectifier coupled to an AC (alternating current) power source. The bridge rectifier supplies pulsating DC (direct current) power to a boost inductor. A pulse width modulator (PWM) driver drives a semiconductor switch, supplying energy to an electrolytic capacitor through a diode. The output of the boost is coupled to a load. A switch, when closed, connects the boost to the AC power source.
One problem that arises is with powering the pulse width modulator driver. The PWM driver is an integrated circuit, and thus will not begin operating until it is supplied with 10 volts DC (direct current). Since the circuit is coupled to a 60 Hz AC (alternating current) voltage source, there will be some amount of time elapsed before the 10 volt DC is supplied to the PWM driver. Until the PWM driver begins operating, reduced power is supplied to the load.
It is highly desirable to have the PWM driver begin operating as soon as possible after the switch is closed. At the same time, of course, the circuit powering the PWM driver must be low cost.
One known method for powering the PWM driver at start up uses current flowing through a resistor to charge a capacitor. The voltage on the capacitor increases until it reaches the turn-on threshold of PWM driver.
After startup, the PWM driver must have a source of higher power. The operation of the PWM driver causes the semiconductor switch to begin operating, causing high frequency current to flow through a boost inductor . The high frequency current is coupled to a secondary winding, rectified by a diode and supplied to a capacitor, thus sustaining the energy in the capacitor at a sufficient level to power the PWM driver. If the switch is a field effect transistor (FET), the total current drawn by the PWM driver and the FET semiconductor switch is approximately 20 milliamps. With a capacitor having a capacitance of 47 mF (microfarads), a startup time of about 0.5 seconds is achieved.
However, if a high voltage, on the order of 800 volts or more, is across the semiconductor switch, then an expensive, high voltage FET must be used. A bipolar junction transistor (BJT) would be more cost effective.
Using a BJT for the semiconductor switch presents an additional problem. Because a BJT requires much more drive current, the amount of current drawn by the PWM driver is much more (on the order of 200 milliamps, as compared to 20 milliamps for an FET).
To supply such a large current, the capacitor must also be larger (approximately ten times larger with a BJT as opposed to an FET). But, if the capacitor is ten times larger, in order to preserve the charging time of capacitor, the resistor must be 10 times smaller. But, if the resistor is ten times smaller, then the power dissipation by the resistor is ten times greater. Such a high power dissipation causes the ballast to become less efficient, since power is being wasted. Additionally, the heat generated by the dissipation in power may adversely effect the operation of the entire ballast.
Thus, a more efficient circuit for quickly energizing the PWM driver is highly desirable.